Three Atlantic threat areas may develop; a record fire season for the U.S.

By: Dr. Jeff Masters , 2:43 PM GMT on August 20, 2012

Share this Blog
59
+

A large tropical wave (Invest 94L) located about 1100 miles east of the Lesser Antilles Islands is headed west at 20 - 25 mph, and is showing increasing organization today. The storm is under light wind shear of 5 - 10 knots, and is over waters of 27°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis and water vapor satellite loops. This dry air is interfering with development, and this morning's visible satellite loop shows that 94L's heavy thunderstorm activity is sparse. However, the satellite loops do show that 94L has now separated from the clumps of heavy thunderstorms to its south, and a pretty well-defined surface circulation has developed. Heavy thunderstorms are now attempting to fire up around this circulation center, but are being hampered by dry air. The center of 94L was about 80 miles to the north of buoy 41041 at 10 am Monday morning, and the buoy recorded SW winds of 10 mph, confirming that 94L probably does have a closed surface circulation. The disturbance will have to build and maintain more heavy thunderstorms than it has now to be considered a tropical depression, though. The first hurricane hunter mission into 94L is scheduled for Tuesday afternoon.


Figure 1. Morning satellite image of Invest 94L.

Forecast for 94L
The latest 8 am EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, for the next five days. Ocean temperatures will warm from 27°C this morning to 28.5°C by Wednesday morning, and the total heat content of the ocean will increase sharply during that period, as well. The main impediment to development will be dry air to the north, and the SHIPS model predicts the amount of dry air will change little over the next five days. I expect that 94L will continue to struggle with dry air through Wednesday, when it will probably have had enough time to moisten the surrounding atmosphere and protect itself against the dry air. The models have shown increasing unity in taking 94L through the Lesser Antilles on Wednesday, and I expect the storm will be a tropical depression or weak tropical storm with 40 - 50 mph winds at that time. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 80% chance of developing into a tropical depression by Wednesday morning. None of the reliable models predict that 94L will reach hurricane strength over the next five days, and it is unlikely that 94L will be able to organize quickly enough to become anything stronger than a 60 mph tropical storm before reaching the Lesser Antilles, given the storm's current struggles with dry air, and the lack of model support for intensification. However, once 94L enters the Eastern Caribbean, wind shear will be low, oceanic heat content high, and the storm should have had enough time to moisten the atmosphere to allow steady strengthening to occur. The main factor that might prevent intensification into a hurricane late this week would be a close pass by the island of Hispaniola. Our top models for long-range 4 - 5 days forecasts all show a path for 94L very close to the island.

Will 94L hit the U.S. mainland?
This storm is a long-range threat to the U.S., as historically, 16% of storms in 94L's location have gone on to hit the U.S., with North Carolina the preferred target (10% chance.) A trough of low pressure capable of pulling 94L to the north enters Western Canada Thursday night, and the exact timing and amplitude of this trough will determine the ultimate landfall location of 94L. The long range 7 - 14 day runs of the GFS model over the past three day have all predicted an eventual landfall for 94L in the U.S., though these long-range runs are notoriously unreliable. The predicted landfall locations have ranged from New England to Texas--which isn't much help. The past three runs beginning on Sunday afternoon have all taken 94L over Florida during the August 27 - 29 time frame, which I'm sure is making organizers of the Republican National Convention uncomfortable, since the convention is in Tampa August 27 - 30. However, 94L could miss Florida completely, as the average error in model forecasts going out 7 days is in excess of 500 miles. We can't rule out a North Carolina landfall, but the pattern we've seen so far this year is for landfalls in Mexico's Yucatan Peninsula, so a more southwards path for 94L into the Yucatan is definitely a possibility. Also, we have that huge drought region in the Midwest, which tends to create its own high pressure bubble, which reduces the odds of storms making the turn and hitting the Central or Western Gulf Coast. If 94L makes it to the Western Caribbean, I see the two most likely options as a landfall in Mexico's Yucatan Peninsula (and then westwards into Mexico south of the Texas border), or recurvature into the Florida Gulf Coast.


Figure 2. MODIS satellite image of Hurricane Gordon taken on Sunday August 19, 2012, at 11:55 am EDT. At the time, Gordon was a Category 2 hurricane with 100 mph winds. Image credit: NASA.

Gordon hits the Azores
The eye of Hurricane Gordon passed over Santa Maria Island in the eastern Azores Islands near 1:30 am EDT this morning. Gordon was a Category 1 hurricane with 75 - 80 mph winds at landfall. Winds at the Santa Maria airport reached a sustained 49 mph at 3 am EDT, but the airport did not report winds during passage of the eyewall at 1:30 am. Reuters reported that Gordon caused only minor flooding and power outages. The hurricane is being sheared apart by strong upper-level winds, and the extratropical remnants of Gordon will not bring any strong winds or significant rain to Europe.

Disturbance 95L in the Gulf near the Texas/Mexico border
A region of disturbed weather (Invest 95L) has developed in the Gulf of Mexico off the coast, just northeast of Tampico, Mexico. The disturbance is due to a trough of low pressure and its associated cold front which moved off the coast over the weekend, but has been fortified via moisture from Tropical Storm Helene, which made landfall Saturday near Tampico. If 95L were to develop into a tropical storm, it would receive a new name. In their 8 am EDT Tropical Weather Outlook, NHC gave 95L a 30% chance of developing into a tropical cyclone by Wednesday morning. An Air Force hurricane hunter aircraft is scheduled to investigate 95L this afternoon. Winds at Tampico this morning were light out of the northeast, which implies that no surface circulation is forming at this time. Radar out of Altamira, Mexico does show some banding to the precipitation echoes, though, which may be indicative of something trying to spin up. The computer models show that 95L should move little over the next few days.


Figure 3. Radar out of Altamira, Mexico at 9:45 am EDT August 20, 2012, shows some banding to the precipitation echoes in association with 95L.

Disturbance 96L off the coast of Africa
The tropical Atlantic is very busy this third week of August, and this is the week of the year that we typically see a major ramp-up of tropical storm activity in the Atlantic. A new tropical wave that emerged from the coast of Africa Sunday (Invest 96L) is headed west at 15 - 20 mph. This disturbance has a modest amount of spin and heavy thunderstorm activity, and is under a moderate 10 - 20 knots of wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 96L a 20% chance of developing into a tropical cyclone by Wednesday morning. This disturbance does not have much model support for development.


Figure 4. The new Suomi National Polar-orbiting Partnership satellite (S-NPP) carries an instrument so sensitive to low light levels that it can detect wildfires in the middle of the night. On August 17, 2012, the Visible Infrared Imaging Radiometer Suite (VIIRS) on Suomi-NPP acquired this image of the wildfires blazing in Idaho. The images were created with data from the instrument’s "day-night band," which sensed the fire in the visible portion of the spectrum. The Halstead Fire, centered about 18 miles northwest of Stanley, was sparked by lightning on July 27, and is burning in an area with large numbers of trees killed by the mountain pine beetle. As of Sunday afternoon, the fire had burned 92,000 acres was only 5% contained, according to InciWeb. The fire prompted the evacuation of the town of Featherville on Saturday night. Red flag warnings for adverse fire weather were posted in the region yesterday, and temperatures reached the low 90s with 16% humidity and winds of 10 mph. Image credit: NASA.

A record fire season in the U.S.
Massive fires continue to burn in Nevada, Idaho and California, and fires that are currently active in the Western U.S. have consumed over 1.3 million acres of land--an area approximately the size of Delaware. Thanks to widespread drought and unusually high temperatures over the past month, 3 million acres have gone up in flames since mid-July, and the fire season of 2012 now ranks in first place for the most acreage burned at this point in the year. According to the Interagency Fire Center, 6.8 million acres have burned as of August 19 this year, beating the previous record set just last year (6.5 million acres for the year-to-date period.) The Interagency Fire Center shows year-to-date records just for the past ten years. The 2012 fire season is well ahead of the pace of 2006, which was the worst fire year in the U.S. for total acreage burned in a year (records began in 1960). In 2006, 9.9 million acres burned, and 6.4 million acres had burned by August 19. With drought conditions far more widespread this year compared to 2006, and the latest forecasts calling for little drought relief over the coming two months, 2012 is likely to surpass 2006 as the worst fire year in U.S. history before the end of the year.


Figure 5. Comparison of drought conditions between the previous record fire year in the contiguous U.S. (2006) with 2012. Drought is much more widespread in 2012 compared to 2006, and 2012 will likely finish ahead of 2006 for the most acreage burned since record keeping began in 1960. Image credit: U.S. Drought Monitor.

Global warming expected to increase fire activity in the Western U.S.
As I blogged about in June, the severe fire seasons of 2012 and 2011 fit the pattern of what we expect to see more of with global warming. Hotter heat waves dry out vegetation more readily, resulting in increased probability of more acreage burned. A study published in the Journal Ecosphere in June 2012 used fire models driven by the output from sixteen climate models used in the 2007 IPCC report and found that while 8% of the planet should see decreases in fire activity over the next 30 years, 38% should see increases. By the end of the century, 20% of the globe should see decreased fire activity, and 62% increased fire activity. In the U.S., the regions most at risk of increased fires are the tundra regions of northern Alaska, and the West, with Arizona and Colorado at particularly high risk.

Jeff Masters

hugh blanket of smoke (got2dogs)
blew in about an hr after my last upload here - I thought I was done for the nite, but this smoke was incredible! made for some awesome light - sooooo eerie!
hugh blanket of smoke
Smoke! What smoke ?? (saltydawgg)
12th Ave road South looking north. Nampa Idaho full of smoke from 7 fires at last count with more dry lightning on the way.
Smoke! What smoke ??
Temecula Fire (photoandy)
This is just two hours after ignition! It quickly became a PYROCUMULUS...
Temecula Fire

Reader Comments

Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

Display: 0, 50, 100, 200 Sort: Newest First - Order Posted

Viewing: 1031 - 981

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | 81 | 82 | 83Blog Index

Quoting MAweatherboy1:
Here's my forecast guys. Sorry if the numbers are hard to read, I've never made one of these before.


Difference with you in forecast intensity,if 94L lands in Dominican Republic as 45 mph storm......... it's over
Member Since: Posts: Comments:
Quoting Tropicalupdate:
U they will call this TD at 5pm or 8pm?

Well, I thought it would be 12 hrs. from 8 this morning, so I'll go with 8 p.m.
Member Since: Posts: Comments:
Quoting Tropicalupdate:
U they will call this TD at 5pm or 8pm?

No..look above. Dr M just posted about the winds being minimal..not even a TD yet
Member Since: Posts: Comments:
1028. Grothar
Quoting Dakster:


Gro- I thought we talked about your scary posting....

I see the tropics are heating up... So which invest gets named first?


I know Nooothing!!! In a way. I am a little jealous of Levi. He got more attention today than when I died last year.
Member Since: Posts: Comments:
Looks to be a "Big Un" for sure.

Member Since: Posts: Comments:
Quoting hurricanehanna:

um...there were variations of N, S, E and W
U they will call this TD at 5pm or 8pm?
Member Since: Posts: Comments:
Thanks, Jeff, for that obs. 94L still has its anticyclone, too.
Member Since: Posts: Comments:
Quoting JeffMasters:
94L is attempting to develop a very large circulation. Take a look at a zoomed-out Central Atlantic loop of the circulation, there's a huge area of heavy thunderstorms to the SW that 94L is trying to wrap in. If it succeeds, this storm will bring heavy weather to a wide swath of the islands Wed and Thu. This may also slow down development, since it takes time to spin up a big chunk of the atmosphere.

Recon into 95L hasn't put out a vortex report yet. They're finding a lot of west winds, but no winds anywhere above 25 mph. Not a TD yet.

Jeff Masters



Thanks Jeff, interesting week ahead for sure. I have to chuckle about your blog on a potential evac of the RNC almost like you knew it was a good possibility.
Member Since: Posts: Comments:
Quoting GTcooliebai:
Also not out of the question for some areas to get absolutely nothing.


Not in this pattern from Tampa north. Just wait till tomorrow and you'll see. As the trough is still digging toward the Gulf. Just ask the people in N FL how it's been todaY AS THAT WEATHER WILL MOVE INTO c fl TOMORROW.
Member Since: Posts: Comments:
Quoting JeffMasters:
94L is attempting to develop a very large circulation. Take a look at a zoomed-out Central Atlantic loop of the circulation, there's a huge area of heavy thunderstorms to the SW that 94L is trying to wrap in. If it succeeds, this storm will bring heavy weather to a wide swath of the islands Wed and Thu. This may also slow down development, since it takes time to spin up a big chunk of the atmosphere.

Recon into 95L hasn't put out a vortex report yet. They're finding a lot of west winds, but no winds anywhere above 25 mph. Not a TD yet.

Jeff Masters



Nicely said..
Member Since: Posts: Comments:
1014. Dr. Jeff Masters, Director of Meteorology (Admin)

Thanks..........
Member Since: Posts: Comments:
94.likes.the.warmer.water
Member Since: September 11, 2010 Posts: 1 Comments: 4598
Authoritative Opinions.

Member Since: Posts: Comments:
Thanks again for joining us, Dr. Masters.
Member Since: Posts: Comments:
1016. Patrap
I never forecast as I'm not a met, but it's a good rule to watch the NHC prog's and discussions and Dr. Masters entries here.

One for the Straight Skinny by the NHC and some insight always from a Phd who flew with the NOAA Hurricane Hunter's is always a great way to stay way High on the "info' curve.


Uploaded by wgbhstocksales on Jul 17, 2009

NOAA plane flies through the eye of hurricane Gilbert in 1988 with a younger Dr. Jeff Masters as the NOAA Flight met aboard.



Member Since: Posts: Comments:
Quoting StormTracker2K:
Folks this is just in 3 days. People from Tampa north better get ready for a serious amount of rain. Not out of the question that some areas may get 10" plus.

Also not out of the question for some areas to get absolutely nothing.
Member Since: Posts: Comments:
1014. JeffMasters (Admin)
94L is attempting to develop a very large circulation. Take a look at a zoomed-out Central Atlantic loop of the circulation, there's a huge area of heavy thunderstorms to the SE that 94L is trying to wrap in. If it succeeds, this storm will bring heavy weather to a wide swath of the islands Wed and Thu. This may also slow down development, since it takes time to spin up a big chunk of the atmosphere.

Recon into 95L hasn't put out a vortex report yet. They're finding a lot of west winds, but no winds anywhere above 25 mph. Not a TD yet.

Jeff Masters
1013. yoboi
Quoting FLASPOTTER:
Where are the afternoon models?



Gro was in here a few min ago.....
Member Since: Posts: Comments:
Quoting Hou77083:


looks like a derecho over the gulf.


Had that kind of a feel to it - we've been getting drenched on the SW panhandle (Pensacola area) since 3AM or so; jut now letting up. Panama City is getting hit hard, and the big blob of storms / moisture is covering the entire northern 1/2 of the peninsula, southern Georgia, and up the coast into SC right now.
Member Since: Posts: Comments:

Member Since: July 25, 2010 Posts: 0 Comments: 2612
1010. LargoFl
SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE MIAMI FL
416 PM EDT MON AUG 20 2012

FLZ067-071-202115-
INLAND BROWARD COUNTY FL INLAND PALM BEACH COUNTY FL
416 PM EDT MON AUG 20 2012

...A SIGNIFICANT WEATHER ADVISORY IS IN EFFECT FOR NORTHERN BROWARD
COUNTY...SOUTHERN PALM BEACH COUNTY...FOR FREQUENT TO EXCESSIVE
LIGHTNING...GUSTY WINDS FROM 45 TO 55 MPH...UP TO NICKEL-SIZED
HAIL...

* UNTIL 515 PM EDT

* AT 414 PM EDT...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A
STRONG THUNDERSTORM 7 MILES NORTHWEST OF CORAL SPRINGS...AND MOVING
NORTH AT 5 MPH.

* THE STORM WILL AFFECT...
RURAL SOUTHERN PALM BEACH COUNTY
AND SURROUNDING COMMUNITIES.

FREQUENT TO EXCESSIVE LIGHTNING...GUSTY WINDS FROM 45 TO 55 MPH...UP
TO NICKEL-SIZED HAIL...TORRENTIAL DOWNPOURS...OR A COMBINATION OF
THESE ARE POSSIBLE. LIGHTNING IS THE NUMBER ONE WEATHER RELATED
KILLER IN FLORIDA. TREES AND OPEN SHELTERS OFFER NO PROTECTION. THESE
WINDS CAN DOWN SMALL TREE LIMBS AND BRANCHES...AND BLOW AROUND
UNSECURED SMALL OBJECTS. SEEK SHELTER IN A SAFE BUILDING UNTIL THE
STORM PASSES.
Member Since: August 6, 2011 Posts: 4 Comments: 38526
1009. Levi32
94L's circulation is still elongated to the east. The models the last few days have been trying to split the eastern lobe off and develop a separate cyclone from it. Those kinds of solutions seem overdone, but that competition just adds to the struggles 94L has had in the central Atlantic. Conditions are improving for it though as we speak. It will be crossing the magic 50W line into warmer water tonight.



Member Since: November 24, 2005 Posts: 635 Comments: 26647
Folks this is just in 3 days. People from Tampa north better get ready for a serious amount of rain. Not out of the question that some areas may get 10" plus.

Member Since: Posts: Comments:
Quoting LargoFl:
..OK ST..thanks for the warning, suns out right now here
Could make it onshore overnight into early morning, I've noticed that trend over the years.
Member Since: Posts: Comments:
Quoting pottery:

Yeah, it's a very large system, and has plenty of associated moisture.
Dry air is to the north, and is our friend !


It's current speed and dry air in the NW quadrant seems to the be the biggest impediments at the moment (we have seen this before recently).............
Member Since: Posts: Comments:
1004. CCkid00
Quoting Patrap:


you are from South Louisiana, right? so...what is your take on 94L. i'm totally just guessing here but it seems like that long line of storms from Mexico to Florida (the front that came through here yesterday), would #1, pick up 95L and just take it with it, and #2, keep anything else from coming our way for the next several days. it would seem that it would actually push 94L eastward, even if it did get in the gulf???
Member Since: Posts: Comments:
floodjam.made.over.50grand.for..n.florida.united.wa ys
Member Since: September 11, 2010 Posts: 1 Comments: 4598
1002. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38526
Quoting RitaEvac:


I've had northerly winds all day here
I had northerly winds on Sunday also, front is in the northeastern counties of south Central Texas currently. THIS AFTERNOON A WEAK FRONTAL BOUNDARY HAS MOVED SOUTHWARD INTO
OUR NE COUNTIES SWITCHING SFC WINDS TO THE N AND LOWERING DEWPOINTS
INTO THE MID 50S ACROSS LEE AND FAYETTE COUNTIES. THIS BOUNDARY
WILL STALL AND BECOME DIFFUSE OVERNIGHT. LOWS TONIGHT WILL RANGE
FROM THE UPPER 60S EAST TO MID 70S ALONG THE RIO GRANDE.

Member Since: Posts: Comments:
1000. LargoFl
Quoting StormTracker2K:


There is a squall line heading for the west coast of FL. Should be here this evening.

..OK ST..thanks for the warning, suns out right now here
Member Since: August 6, 2011 Posts: 4 Comments: 38526
Quoting Chicklit:

That's a lot of energy.


sorry have to post it again.
awesome.


looks like a derecho over the gulf.
Member Since: Posts: Comments:
Patrap maybe it time for another tutorial on the purpose of the ignore button, unless you feel you'll be ignored.
Member Since: Posts: Comments:
Well defined circulation with dry air entrancement very evident.

Member Since: Posts: Comments:
Quoting weathermanwannabe:
The big picture vis loop really gives you an idea of how large the circulation of 94L is......That will take some time to consolidate.

Link

Yeah, it's a very large system, and has plenty of associated moisture.
Dry air is to the north, and is our friend !
Member Since: Posts: Comments:
Baha..well said!..



Im out till I come back..behave!
Member Since: Posts: Comments:
Quoting sar2401:


That's not the front. The actual front is stalled to the north and spinning off shortwave troughs. It's been doing this for three days. The only difference is that some ennergy from 95L appears to be joining the parade. BTW, looking at the frontal map, what's that big low off South Carolina doing there?



Edit: Never mind, the supposed low is gone. Apparently some kind of artifact that snuck into the computer. :)
Member Since: Posts: Comments:
Baha, what are your thoughts on this potential problem for us? :s
Member Since: Posts: Comments:
Look how deep the trough has dug over the CONUS, usually with this the recurve would be inevitable. I wonder why this time is different.

Member Since: Posts: Comments:
Quoting Chucktown:


I have to, well, until someone names their local rock band after me !!


I'll get to work on that....
Member Since: Posts: Comments:
Member Since: August 6, 2011 Posts: 4 Comments: 38526
Member Since: Posts: Comments:
Quoting hurricanehanna:

um...there were variations of N, S, E and W


here is the link to the site
Link
Member Since: Posts: Comments:
Quoting LargoFl:
well boomers stopped here and not a drop of rain, looks like everything is going northward not eastward as is usual


There is a squall line heading for the west coast of FL. Should be here this evening.

Member Since: Posts: Comments:
Quoting Bluestorm5:
Look, folks... Levi is not the only one making forecast. I made these two forecasts and I didn't get bashed for them. Why is Levi being bashed for more educated forecast and not me for posting such a stupid forecast?




hmm nice cone ;)

94L is looking really good, circulation has gotten much more better define, convection is increasing on the SW quad of the LLCOC and spreading in toward the center of the LLCOC. convection is also increasing within the bands to 94L's NW, W, E, SE, also bands are becoming thicker. I think it will be upped to either 90% or 100%, with advisorys starting at either 5pm, 8pm, or 11pm, and maybe watches for the lesser antilles.

also they may move up Hunter's mission time to tomorrow morning or midday, instead of afternoon, as it is currently slated for.
Member Since: Posts: Comments:
Where are the afternoon models?
Member Since: Posts: Comments:
there is a vigorous spin to the south of the apparent centre of 94L. Is this a centre relocation? that strog vortex is moving wsw.
Member Since: Posts: Comments:
The big picture vis loop really gives you an idea of how large the circulation of 94L is......That will take some time to consolidate.

Link
Member Since: Posts: Comments:
Quoting Tropicalupdate:
what were the winds?

um...there were variations of N, S, E and W
Member Since: Posts: Comments:
Member Since: Posts: Comments:

Viewing: 1031 - 981

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | 81 | 82 | 83Blog Index

Top of Page

About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

Local Weather

Overcast
70 °F
Overcast